1. Field of the Invention
The present invention relates generally to nuclear fuel rods and, more particularly, is concerned with a method of welding a gas pressurization axial passage in an end plug and a trepan configuration provided on the end face of the end plug.
2. Description of the Prior Art
In a typical nuclear reactor, such as a pressurized water type, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between the nozzles and a plurality of transverse support grids axially spaced along and attached to the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Each fuel rod includes a plurality of nuclear fuel pellets loaded into a cladding tube and bottom and top end plugs applied to opposite ends of the tube to hermetically seal the tube. The bottom and top end plugs are typically girth welded to the opposite ends of the tube. Ordinarily, the top end plug also contains an axial gas pressurization passage or bore. An axial electrode is used to form an end seal weld on the end face of the top end plug to close the axial bore after pressurization of the tube with an inert fill gas.
It is well established that, when using TIG (tungsten inert gas) welding, the arc tends to start on external corners of the end plug due to the concentration of ions at these points. Since the typical top end plug has a flat end face, arc initiation and location of the arc is not always on the center of the plug. It is desirable for the arc to strike as close on true center as possible to produce a high quality weld.
Consequently, there is a need for improvements which will promote better centering, uniformity, and depth of the end seal weld on the top end plug.